The most common problem associated with RCDs is that of nuisance tripping, and in the majority of cases this is due to the RCD tripping in the absence of a sustained residual current above a predetermined level. As used herein, the term “sustained residual current” refers to any residual current which continues to flow until it is interrupted by removal of power, whereas “non sustained residual current” refers to any current which automatically ceases to flow after an initial surge or impulse. A non sustained residual current can flow to earth in response to lightning surges, switching of reactive loads or momentary current flows to earth which are not attributable to insulation breakdown and therefore not sustained. Nonetheless, in many cases such currents will be perceived by the RCD as a residual current of sufficient magnitude and duration such as to cause the RCD to trip.
For reasons of safety and optimum protection, many RCDs have an inverse time/current characteristic which causes the RCD to trip faster for higher magnitude residual currents. However, this characteristic may predispose the RCD to trip automatically in response to the flow of residual currents of high magnitude but short duration. In the case of lightning surges, the resultant current flow to earth can have a magnitude in the amperes region, and although such currents may flow for only a period of a few microseconds, the RCD may unavoidably trip. The nuisance tripping problem has recently been compounded by the use of electronic starters in fluorescent lights where an ignition current of hundreds of amperes with a duration of a few milliseconds can result in the flow of relatively large capacitive currents to earth for a similar period. Conventional RCDs have little immunity to such currents and are highly likely to nuisance trip.
RCD designers have tried various means to make their products immune to such problems, such as the addition of mechanical slugging and time delay circuitry. However, the amount of slugging or time delay that can be added to an RCD is limited because of a requirement to trip within specified time limits for different magnitude fault currents. Typically, a general type 30 mA RCD complying with RCD product standard IEC61008 will have to trip within 300 mS for a residual current of 30 mA and within 40 mS for a residual current of 150 mA or higher. This type of response to higher magnitude residual currents provides the inverse time/current characteristic. Excessive slugging or time delay will make it impossible for the RCD to meet the 40 mS requirement, so the effectiveness of such improvements is limited.
In addition, the use of electronic circuitry to achieve a time delay often results in the storage of charge in reactive components which can have the effect of stretching the duration of the signal as seen by the electronic circuitry. In effect, because of the large magnitude of the residual current, the resultant accumulative storage effect in reactive components will cause the RCD to perceive a residual current flow of a substantially longer period than the actual period of residual current flow.